1. Field of the Invention
The field of the invention is that of seismic data acquisition. More precisely, the invention relates to seabed analysis equipment.
In particular, the invention relates to the seismic oil exploration industry, but can apply to any field implementing a seismic data acquisition network in a marine environment.
2. Related Technology
On-site geophysical data acquisition operations conventionally implement sensor arrays (designated by the term “hydrophones,” in connection with data acquisition in a marine environment).
In order to collect geophysical data in a marine environment, one or more underwater seismic sources are activated in order to propagate omnidirectional seismic wave trains.
The sources currently being implemented to conduct seismic surveys are air guns.
The wave trains generated are sensed by the aforementioned hydrophones, which are distributed along cables in order to form linear acoustic antennas commonly designated by the term “streamer”.
Acquisition of seismic data in an environment is conventionally carried out with the assistance of a series of streamers towed by a vessel.
Each streamer may include a head buoy and a tail buoy with which means for global positioning by satellite are associated, for the purpose of accurately tracking each stream.
This tracking of the streamers is important, in particular for:                following the position of the hydrophones in order to obtain an adequately precise image of the seabed;        detecting the movements of the streamers in relation to one another;        following the navigation of the streamers, in particular in the situation wherein an obstacle such as an oil barge is circumnavigated.        
It is noted that streamers include an assembly of sections generally having a length of approximately 150 meters, the streamers having a total possible length of several kilometers (conventionally 6 to 7 kilometers).
In actual practice, it is sought to conduct analysis of a seabed with a minimum number of passes of the vessel over the zone concerned. To accomplish this, the width of the sensor array is increased as much as possible, which involves the implementation of a significant number of streamers.
The tracking of streamers is therefore a particularly sensitive issue, taking into account the length and number thereof.
As a matter of fact, the streamers are subjected to various external stresses varying in nature and importance, such as the wind, waves, currents . . . .
These stresses regularly result in relative movements of the streamers, at the risk of them becoming entangled with one another, which can cause more or less consequential damage to the streamers.
Currently, one solution attempting to control the respective positions of the streamers is based on the implementation of navigational control devices (commonly designated by the term airplane or “birds”) such as those described in the patent document published under the number FR-2 870 509.
These devices include a body equipped with pivoting wings enabling the position of the streamer to be modified laterally.
Furthermore, the “birds” can be equipped with pressure sensors for detecting variations in depth and for bringing the streamer to a predetermined depth.
Additionally, the set of “birds” is piloted by a centralized system such as the one described in the patent document published under the number WO-02/103393.
According to this technique, active controllers (acoustic transducers, GPS devices . . . ) are evenly distributed along the streamers and the signals supplied by these controllers are transmitted to a master controller present on the survey vessel.
The master controller centralizes and processes the data for the purpose of comparing it to a predetermined configuration. To accomplish this, the active controllers, and in particular the acoustic transducers, are labelled as cells: one cell includes a transducer considered to be central for the cell in question, and of several peripheral transducers on the streamer or streamers immediately adjacent to the one carrying the central transducer, distances being established for each position of the peripheral transducers relative to the central transducer.
On the basis of the comparison result between the predetermined configuration and the actual configuration, the master controller returns instructions to the birds distributed along the streamers for the purpose of modifying the positions thereof.